This invention relates generally to steam turbines, and more particularly to improved apparatus for controlling a flow of steam to such turbines.
In a steam turbine generator system, the turbine is normally maintained at a constant speed and steam flow is varied to adjust the torque required to meet the electrical load imposed on the generator. This type of control is provided by a main control system which varies the flow of steam to the high-pressure turbine, and in some instances to the low-pressure turbine, to meet the load demand. The main control system is designed to accommodate for normal changes in load demand and to smoothly adjust the turbine operating conditions to the new demand. However, if the electrical load is suddenly lost or reduced significantly, a commensurate reduction must be made in the flow of steam through the turbine or the turbine will overspeed, possibly causing turbine damage. The main control system does not possess sufficiently rapid response characteristics to accommodate for such sharp variations in low demand, especially in high power to inertia ratio turbine systems.
As is well known, large steam turbines generally include multiple nozzle chambers through which steam is directed into the turbine nozzle through turbine blades which are rotated thereby. Nozzle chamber activation (i.e., steam admission thereinto) is regulated by valves which open to provide steam flow from steam supply conduits into the nozzle chambers, and close to obstruct steam flow thereinto. A valve point is defined as a state of steam admission in which each valve is in the completely open, unobstructing configuration. As is well known, in actual operations of conventional steam chests the valve point does not occur at a full open or full closed position, but occurs just prior to the actuation of the next valve. It can be shown that maximum turbine efficiency can be obtained from the use of an infinite number of valve points which, in turn, requires an infinite number of valves.
Of course, a finite number of valves must be used on steam turbines with that number of valves being dictated by compromises between improved turbine performance and increasing capital cost for increasing numbers of valves. One or more valves control the flow of steam into each nozzle chamber. Nozzle chamber activation refers to the process of increasing steam flow into the nozzle chambers from the time steam flow threinto is initiated until the maximum steam flow thereinto (i.e., completely activated) is achieved. Deactivation refers to the process of decreasing steam flow into the nozzle chambers. When multiple valves are used to regulate steam flow into a single nozzle chamber, those valves typically modulate together. Since such valves modulate together, turbine efficiency is actually a maximum when the nozzle chambers are each in the completely activated or completely deactivated. Heretofore, the nozzle chambers were activated in a predetermined sequence such that once the nozzle chamber was activated during increasing load on the turbine, it was not deactivated until the load on the turbine decreased. One of the few restraints on nozzle chamber activation sequence was that single shock operation was preferred over double or multiple shock operation. That is, it is usually preferable practice to activate nozzle chambers such that newly activated nozzle chamber (i.e., after minimum admission) is circumferentially adjacent at least one previously activated nozzle chamber. One illustrative method for admitting steam into a steam turbine is disclosed in U.S. Pat. No. 4,325,670, issued Apr. 20, 1982 to George J. Silvestri, Jr., assigned to the assignee of the present invention, and incorporated herein by reference.
One recurring problem encountered by such turbines, however, is known in the art as low cycle thermal fatigue. With many older turbines being relegated to cycling operations such as load following and on-off or "two shifting" operation, the potential for low cycle thermal fatigue is increased significantly. The problem of low cycle thermal fatigue can be minimized in newer turbines by placing individual actuators for each valve in the steam chests of the turbines. Older steam chests, such as those used in the mechanical hydraulic (MH), analog as those used in the mechanical hydraulic (MH), analog electric hydraulic (AEH) and digital electric hydraulic (DEH) turbine control systems, may not have individual valve actuators, nor may they have sufficient s pace between the valves to accommodate individual valve actuators. This is especially true in those cases where the actuator incorporates springs necessary to insure rapid closure of the valves during turbine trips. One solution to such problems would be the wholesale but costly replacement of the steam chests. It would, therefore, be desirable to modify existing steam chests to minimize low cycle thermal fatigue caused by cycling operations.
It is well known that low load and part load operation of steam turbines with sliding throttle pressure not only reduces low cycle thermal fatigue, but also improves the heat rate. In particular, operation in a hybrid (i.e., a combined mode of operation with constant pressure-sequential valve and sliding throttle) results in a maximum heat rate benefit while reducing the change in first stage exit temperature, thereby reducing low cycle thermal fatigue. With hybrid operation, a partial-arc admission turbine is operated in the upper load range by activating individual valves to effect load changes along with constant throttle pressure operation. As load is reduced, when a particular valve point is reached, valve position is held constnat and throttle pressure is varied or slid to achieve further load reductions. On units with essentially 100% admission at maximum load, hybrid operation with a 50% minimum first stage admission achieves the heat rate benefit of constant throttle pressure operation. Additionally, when valve loop losses are considered, hybrid operation has superior thermal performance to partial-arc designs operating with constant throttle pressure and having admission points below 50% at loads below from 65 to 70% of a maximum value. For units with considerably less than 100% admission at maximum load, optimum hybrid operation is achieved at the valve point where half of the valves are wide open and half are closed Therefore, it would be desirable to provide apparatus for a valving sequence on turbines having steam chests without individual actuators in such a manner that the valves correspond to 50% first stage admission (or half of the total number of valves) all open simultaneously, thereby achieving optimum hybrid operation.
However, start up procedures that increase rotor life require a different operating mode than hybrid operation. Full-arc admission during turbine roll, for example, has proven beneficial for rotor warmup and more uniform heating as well as reducing the steam-to-metal temperature mismatches that increase low cycle thermal fatigue. It has also been noted that maintaining full-arc admission operation beyond synchronization of the turbine up to some level of load can be beneficial. Full-arc admission operation at part load, however, cannot be achieved on turbines having steam chests without individual valve actuators for which the valves are set for minimum first stage admissions below 100%. It has also been noted that an expected increase in rotor life is achievable when the transfer from full to partial-arc is made during the loading cycle as compared to full-arc admission operation all the way to full load. It is, therefore, apparent that a steam chest having the capability of valve transfer from full to partial-arc admission and vice versa would be extremely desirable for turbines utilized in cycling operations.